The present invention relates to a method and apparatus for cleaning a stream of gas, and particularly for removing dry particulate matter from a stream of gas.
Maintenance of air purity is a significant problem in many industries in which dust is necessarily produced during manufacture of products. For example, sawdust and sander dust are produced in great quantities in the wood products industry, and dust is inevitably produced during handling of grain products.
In many cases the dust removed from a stream of gas is of commercial value. For example, the wood dust resulting from sander or sawmill operation is usable as furnace fuel or as a component of particle board and similar composition material. Similarly, much of the dust produced during handling of grain contains nutrients usable as animal feed, if not for human consumption. It is therefore desirable to retain these materials, rather than discard them.
Many methods and types of apparatus have been developed to filter particulates and other suspended matter from streams of gas. Known methods, however, present some significant drawbacks. For example, cyclone separators are commonly used to remove the bulk of dry particulate material from a stream of gas. The amount of such dust which can be removed from the air by a primary separator such as a cyclone separator, however, is frequently insufficient, and a secondary separator must be used to remove additional lighter or smaller particles from the stream of air. In known apparatus, the matter collected in the secondary cleaning apparatus frequently must be handled separately from that collected in the primary apparatus. This is particularly true when secondary cleaning apparatus involves the use of liquid sprays to remove particulates from suspension.
The particulate matter collected in a wet secondary filter must ordinarily be separated from at least a large part of the liquid, to conserve the liquid and facilitate handling. Unless substantially all moisture has been removed such material has previously needed to be handled separately, requiring extra labor and causing inconvenience.
Apparatus used to transport light and fine particulate materials is typically subject to damage from invasion of the particles into bearings and motor cases. It is therefore desirable not to handle this material separately.
When particulate solid materials have been removed from the air by exposure to a liquid spray they have previously required additional handling, because the particulate matter is dispersed in a relatively large quantity of liquid. Introduction of a small amount of particulate matter, along with a large amount of liquid, into the large quantity of dry particulate matter separated from a stream of gas in a primary separator is unsatisfactory because it would add unacceptable amounts of moisture to dry material such as sander dust removed from a stream of gas, making it susceptible to spoilage and difficult to handle, or worthless for later use.
Some types of combustible dry material, such as fine sander dust, grain dust, and similar materials, form explosive mixtures with air and are thus potentially dangerous to handle. It is well known that grain elevators and flour mills are too frequently the sites of dangerous explosive fires, and a similar hazard is presented by very dry sawdust.
Multi-stage apparatus for removing dry particulate material from a stream of gas is shown in McGregor et al, U.S. Pat. No. 3,203,156, and apparatus for removing suspended wet particles from a stream of air is shown in Skoli et al, U.S. Pat. No. 3,771,289. Selmeczi, U.S. Pat. No. 3,914,378, and Hall et al, U.S. Pat. No. 3,268,296 both disclose gas scrubbers incorporating sprays impinging on both sides of a filter element, but no provision is made for periodic backwashing of the filter element. Furthermore, none of these patents provides for continuous collection of all material separated from a stream of gas at a single point.
What is needed, then, is a method and apparatus for providing primary and secondary removal of dry particulate matter from a stream of gas and making a maximum amount of commercially valuable particulate matter so recovered available at a single point of collection. It is also desirable to reduce the risk of explosion caused by presence of such dry combustible particulate matter in a stream of air.